x86, microcode: Don't request microcode from userspace unnecessarily

Requesting the microcode from userspace *every time* when onlining CPUs
(during a CPU hotplug operation) is unnecessary. Thus, ensure that
once the kernel gets the microcode after booting, it is not freed nor
invalidated when a CPU goes offline, so that it can be reused when that
CPU comes back online, without requesting userspace for it again. As a
result, the CPU hotplug operations become faster as well.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Link: http://lkml.kernel.org/r/4E91F908.5010006@linux.vnet.ibm.com
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/kernel/microcode_core.c b/arch/x86/kernel/microcode_core.c
index f9242800bc84..f2d2a664e797 100644
--- a/arch/x86/kernel/microcode_core.c
+++ b/arch/x86/kernel/microcode_core.c
@@ -483,7 +483,13 @@ mc_cpu_callback(struct notifier_block *nb, unsigned long action, void *hcpu)
                 sysfs_remove_group(&sys_dev->kobj, &mc_attr_group);
                 pr_debug("CPU%d removed\n", cpu);
                 break;
-        case CPU_DEAD:
+
+        /*
+         * When a CPU goes offline, don't free up or invalidate the copy of
+         * the microcode in kernel memory, so that we can reuse it when the
+         * CPU comes back online without unnecessarily requesting the userspace
+         * for it again.
+         */
         case CPU_UP_CANCELED_FROZEN:
                 /* The CPU refused to come up during a system resume */
                 microcode_fini_cpu(cpu);